Increasing demand for enhanced operational performance in a variety of commercial, industrial, and military products has paved the way for development of the next generation of high performance materials. Recent efforts in this regard have focused on nanomaterials due to their unique properties that are not usually observed in equivalent macroscale materials. Nowhere has the focus on nanomaterials been more profound than in the composite field, where the beneficial properties of the composite matrix and the nanomaterials can be effectively combined to produce a composite material exhibiting desirable properties of both components. Although many types of nanoscale composite materials have been prepared, electrically conductive polymer composites not based upon traditional metallic conductors have proven to be among the most studied for immediate practical applications.
Although lead has been traditionally used in numerous industrial applications, current regulations have mandated the phase out of lead in most commercial products. For example, the European Union issued regulations in 2006 that mandated the elimination of lead from coatings and solders used in most electronic components. Other countries have issued similar mandates.
Soldering applications, particularly in electronics and vehicle manufacturing, have been heavily impacted by the ban on lead. Many alternatives to traditional lead-based solder materials have been developed, In/Sn, Ga/Sn, Bi/Sn and Sn/Ag/Cu (SAC) alloys being among the most widely used. Although most lead-free solder materials are slightly more difficult to use than are traditional lead-based solder materials, the lead-free solder materials have typically proven satisfactory for most commercial applications. However, for certain high performance applications, lead-free solder materials have failed to tolerate commonly encountered operational conditions. Non-limiting examples of conditions under which lead-free solder materials can be unsatisfactory include, for example, extremely low temperatures and environments having thermal shock and high stress conditions. In addition, lead-free solder materials are prone to exhibit a coefficient of thermal expansion (CTE) mismatch with a substrate to which they are attached, ultimately leading to failure of the solder material. These limitations and others have limited the use of lead-free solder materials in aerospace and military applications, in particular.
A number of different metal nanoparticle compositions have been proposed for the next generation of lead-free solder materials. As the size of metal nanoparticles drops below about 20 nm, their melting point drops precipitously, thereby allowing certain metal nanoparticles such as, for example, copper and tin to be flowed at temperatures that are comparable to those of traditional lead-based solder materials (e.g., less than about 200° C.). However, once neat metal nanoparticles have been flowed to form a connection, they become at least partially fused together to form larger particles, at which stage they experience a rapid rise in melting point, approaching that of the corresponding bulk metal. Whereas the initial formation of a connection using metal nanoparticles can be extremely facile, rework can become quite problematic due to the much higher reflow temperatures typically needed after initial processing.
In view of the foregoing, lead-free, electrically conductive compositions that can be reworked and have enhanced performance under extreme conditions are needed in the art. The present invention satisfies this need and provides related advantages as well.